Low-Complexity Adaptive Background Compensation of Coherent Optical Transmitters

Next generation optical communications will demand coherent transceivers with data rates in the 1.6 Terabit-per-second range and beyond, requiring both high symbol rate and large constellations. In this scenario coherent systems become increasingly sensitive to transmitter imperfections such as In-phase/Quadrature time skew, amplitude and phase errors, bandwidth limitations and mismatches, etc. These impairments must be compensated to achieve satisfactory performance. In this work, we propose a novel and efficient technique for the precompensation of such impairments, requiring minimum additional hardware in current transmitter designs. This scheme is
adaptive
, as it readjusts itself to cope with time and environment dependent impairments, and
background
, as interruptions of normal operation are not required for the continuous recalibration of the system. Naturally, such
in-field
adaptation demands the real-time system identification of the coherent optical transmitter, which is performed from the sole information provided by a low bandwidth photodiode/analog-to-digital converter chain at the transmitter output. This fact represents a meaningful advance with respect to the usual channel estimation methods, which require the full bandwidth coherent demodulation of the optical output. A theoretical foundation for the proposed scheme is presented. Numerical simulations of a realistic coherent optical transmitter show excellent results for high symbol rate modulation formats.